Know Your Hydrodynamics?

When calculating pressure drop in a liquid system, you look at the total equivalent length of your piping system. This includes your length of pipe (say 100 feet of 6"), but also the equivalent length of fittings, such as valves, elbows, tees etc.

For example, for 6" pipe:

- an open gate valve is equivalent to 4.04 ft of pipe
- a 90 degree elbow is equivalent to 15.2 ft of pipe
- a branch flow tee is equivalent to 30.3 ft of pipe
- a check valve is equivalent to 50.5 ft of pipe.

Without knowing your pipe diameter and length, the required discharge pressure, the depth of the submerged pumps, and what fittings you have, the question cannot be precisely answered. Also, you haven't included anything on how you are controlling your discharge system pressure.

However, a branch flow tee is probably not going to add that much (percentage-wise) equivalent pipe length to your system. I'd say your flow will not be much below 600 gpm.

Hmm...ok. I don't have one particular application in mind, thus the generic question. But, I can give two hypothetical examples...

One, would be pumps A & B feeding a filtering device, where the output pipes from the pumps are 4" long till the 90° turn, then another 4" to the T, then about 6" from there up to the filter. All pipes hare are .75 ID.

Two, would be pumps A & B drawling from a filtration device that sits on the floor, with the same 4" to the 90° turns, same 4" to the T, but would be pushing about 50" up to the reservoir. In this case, the shorter pipes would be .75 ID, and the 50" vertical pipe would be .75 or 1.5 ID, which ever would be better.

In exp. 1, the combined output pipe has to be .75 ID, as the filtration device's intake port will only accept .75 ID.

Simple hydraulics..
The end result will be 600 GPM.. All that would change would be the pressure due to friction loss in the pipe, the elbows and any appliances..

None of those will effect volume but will effect net pressure..

It wouldn't matter if the pumps were pumping 300 gpm at 50psi or 100psi.. it is still 300gpm each or 600 in total !!

BTW, I am a Deputy Fire Chief and hydraulics are right up my alley !!



Murray

When pumping water your pumps will almost certainly be centrifugal. All centrifugal pumps operate on a pump curve that the vendor should supply. The pump will have a design point (say 100 gpm at 50 psig differential pressure) that is around halfway on the curve. Where the pump actually operates on the curve depends on the total friction losses in your system, but is usually set by a pressure control valve (PCV) or a flow control valve (FCV) installed, at least in industial settings.

The pump curve will have flow, usually in gpm, on the x-axis. The y-axis is proportional to the differential pressure across the pump (discharge pressure minus suction pressure). The curve is always a downward arc from left to right. For the example of the design point at 100 gpm at 50 psi, the curve would be something like 0 gpm and 70 psig (the discharge valve closed), 25 gpm and 65 psig, 100 gpm and 50 psig (the design point) and 125 gpm and 35 psig. Connect the points to draw the operating curve. The pump will always be somewhere on this curve (unless you have worn impellers, etc).

For the cases you've described, 10 gpm in 3/4" schedule 40 carbon steel pipe has a pressure loss of 10.2 psi per 100 feet of pipe. For the discharge of a single pump, 5 gpm would be a 4.5 psi pressure drop over 100 feet of pipe. The equivalent length of fittings for 3/4" pipe are:

gate valve 0.55 feet
90 degree elbow 2.06 feet
branch flow tee 4.12 feet
check valve 6.86 feet

Also, most industrial filters I've worked with are designed to have the element replaced when the pressure drop reaches 10 psi.

It looks like you'll have a maximum pressure drop of about 20 psi. As long as your pumps are designed to have a differential pressure above 20 psi, you should see no reduction in flow rate due to line loss.

In industrial settings, you control your pumps one of two ways:

1. You have a PCV downstream of the pump. This holds the pressure constant while letting the flow vary.

2. You have a FCV downstream of the pump. This holds the flow constant while letting the pressure vary.

(you can also have a level control valve, which holds the liquid level in an upstream or downstream vessel constant, and lets both the flow and pressure vary).

For your situatiuon, it sounds like you're letting the total line friction loss (length of pipe + equivalent length of fittings + pressure drop across the filter) determine where on the pump curve you will operate.

Hope this was of some help to you.

Yes, indeed! Thanks to you both!

All piping produces a drag (friction loss)
All piping produces a drag (friction loss).
When two flows join to for a third flow as in your picture there will be a restriction (drag, friction loss, etc). The design or shapes and size of pipes will govern the loss of flow. You won't get 600 gpm at termination of the pipes. It could be close but transitions produse losses (no propetual motion machine here).

There is a friction loss, that is taken.. BUT, there is NO loss of volume !!!

Better check your hydraulic calculations again !!!

Murray

This is a trick question of sorts.
If both punps are actually pumping 300 gpm then 600 gpm will exit.

Most if not all pumps are rated by their manufactures as to gallons per minute (gpm). Its a nominal rating under conditions of a given suction head (lift) and discharge losses if any. Most if not all 300 gpm pumps will not pump 300 gpm in actual use. The 300 gpm rating may be from a zero suction lift and zero losses at discharge, etc..

Normally a pump rated 300 gpm will pump no more than 300 gpm and doesn't enclude losses for discharge as shown in your picture.

Every transition or length of pipe will reduce flow (gpm), a fact....